retrorocket

or retro-rocket

a small, auxiliary rocket engine, forming a part of a larger rocket vehicle and having its exhaust nozzle pointed toward the direction of flight, for decelerating the larger rocket, separating one stage from another, etc.

retrorocket

/ˈrɛtrəʊˌrɒkɪt/

noun

1.

a small auxiliary rocket engine on a larger rocket, missile, or spacecraft, that produces thrust in the opposite direction to the direction of flight in order to decelerate the vehicle or make it move backwards Often shortened to retro